1. Field of the Invention
This invention relates to a method of reducing the compression set of silanol-containing silicone elastomer bases and of special interest are fluorosilicone elastomer bases.
2. Background Information
Compression set in rubber is the residual deformation of the rubber after removal of the compressive stress. Compression set is measured thirty minutes after removal from the loading device in which the specimen had been subjected for a definite time to compression deformation under specified conditions of load application and environment. Compression set is an important property for many applications of rubber, such as being used as a gasket to seal a fuel line, a fuel tank, and other equipment which might be used for distribution and holding of fuels. Fuels and solvents are very harsh environment for rubber products and certain types of rubbers have been developed for use as gasketing material, hose material, and bladder materials which come into contact with fuels and solvents. One such rubber is fluorosilicone rubber which has excellent resistance to deterioration by contact with fuels and solvents. However, fluorosilicone rubber has suffered from a high compression set which is a negative characteristic for use in many applications, such as in gasketing applications. As so often happens with materials where one wishes to improve one property, improving that property results in the decrease in one or more other properties, so that the final material chosen usually ends up in a compromised property profile.
Fluorosilicone rubber has been known for many years and has been used in the applications of fuel gaskets in spite of the high compression set. Fluorine-containing polydiorganosiloxane gums are known from Johannson in U.S. Pat. No. 3,002,951, issued Oct. 3, 1961. Johannson teaches a method of preparing high molecular weight gums having perfluoroalkyl radicals on silicon from cyclic siloxane trimers. The gums prepared by Johannson's method are hydroxyl endblocked and are proposed as suitable for the preparation of rubber, a fluorosilicone rubber. Johannson is hereby incorporated by reference to show gum of perfluoroalkyl containing siloxanes and their preparation. Other gum polymers containing the perfluoroalkyl radical are shown by Hyde et al in U.S. Pat. No. 3,274,153, issued Sept. 20, 1966; McVannel in U.S. Pat. No. 3,294,740, issued Dec. 27, 1966; and Brown in U.S. Pat. No. 3,373,138, issued Mar. 12, 1968 which are all hereby incorporated by reference to show hydroxyl endblocked polymethyl(3,3,3-trifluoropropyl)siloxane gums and their preparation. Razzano in U.S. Pat. No. 3,997,496, issued Dec. 14, 1976, teaches a method of making silanol end-stopped perfluoroalkyl-substituted organopolysiloxane homopolymer or copolymers by using a non-bond-rearranging catalyst starting with low molecular weight silanol polymers. Razzano also teaches that the molecular weight of the silanol end-stopped perfluoroalkyl-substituted organopolysiloxanes can be regulated by using a very low molecular weight trimethylsiloxy and silanol end-stopped polydimethylsiloxane, where the more regulator used, the lower the molecular weight of the resulting polymer. The highest molecular weight polymers are obtained without the use of regulator. Similar chain-stoppers are described by Razzano in U.S. Pat. No. 4,341,888, issued July 27, 1982, in his method of making polydiorganosiloxanes.
Bluestein in U.S. Pat. No. 4,267,298, issued May 12, 1981, teaches that the preparation of triorganosilyl end-stopped fluoro substituted polysiloxane fluids are very difficult to make. If one attempts to use a triorganosilyl chain stopper or regulator while making the polymers, one either proceeds directly to a high gum polymer without the chain stopper being polymerized into the polymer or the polymer does not form but only cyclotetrasiloxanes form. Bluestein teaches that a silanol endblocked polydiorganosiloxane is reacted with a triorganochlorosilane to give the endblocked polysiloxane fluids.
Bluestein et al in U.K. Patent Application No. 2,118,960, published Nov. 9, 1983, teach that silanol or aliphatic alcohol chain-stopped fluorosilicone polymers can be compounded into elastomer compositions which cure to elastomers with compression set values such as 36% (347.degree. F./22 hours) and by the use of rare earth octoates as compression set additives can be 39.0% for silanol chain-stopped (350.degree. F./22 hours) and 34.1% for trimethylsiloxy chain-stopped (350.degree. F./22 hours).
Brown in U.S. Pat. No. 3,179,619, issued Apr. 20, 1965, teaches low swell organosiloxane rubbers containing fluorinated aliphatic radicals attached to the silicon atom of the polysiloxane gum chain. Brown teaches that the use of limited amounts of vinyl containing siloxane units in the gums gives elastomers of superior compression set. Brown reports tension set values in his Example 1, but does not report any compression set values. Brown also teaches that fluorosilicone rubber with tensile strengths of 1,200 psi with elongations of 540 percent can be made using a polysiloxane gum of methyl(3,3,3-trifluoropropyl)siloxane units, fume silica, and benzoyl peroxide.
Talcott in U.S. Pat. No. 3,006,878, issued Oct. 31, 1961, teaches the preparation of fluorinated silicone rubber compositions based on gums such as described by Johannson. These fluorinated silicone rubber compositions described by Talcott are prepared by base heating a mixture of 3,3,3-trifluoropropylmethylpolysiloxane gum, silica filler such as fume silica, and 3,3,3-trifluoropropylmethylsiloxane fluid which is hydroxyl endblocked. Talcott teaches that the use of limited amounts of vinyl containing siloxane units in the gum gives elastomers of superior compression set, as was taught by Brown in U.S. Pat. No. 3,179,619, filed June 27, 1956. No values for compression set are reported for the compositions described in Talcott.
Pierce in U.S. Pat. No. 3,122,521, issued Feb. 25, 1964, teaches a method of polymerizing fluoroalkyl-substituted cyclotrisiloxanes to make high molecular weight polymers. Pierce teaches that using these polymers to make compositions which cure to silicone rubber with improvement in the tear strength and an occasional improvement in the tensile strength. One fluorosilicone rubber, after curing for 24 hours at 150.degree. C. has a tensile strength at break of 1,770 psi and an elongation at break of 340 percent. The gum used to make this fluorosilicone rubber was made from 99.6 mol percent 3,3,3-trifluoropropylmethylsiloxane units and 0.4 mol percent methylvinylsiloxane units with a Williams plasticity of about 0.250 inch. Pierce reports tear strengths of 117 pounds per inch, but no compression set values are reported.
Elias et al in U.S. Pat. No. 4,116,919, issued Sept. 26, 1978, teach a treated reinforcing silica which can be used to make silicone elastomers with improved durometer and tear strength. The silica is treated with a mixture of a hexaorganodisilazane and bis-Si-(1-methyl-1-silacyclopentenyl)amine. The silica can be treated either in situ during the manufacture of curable silicone elastomer compositions or as a separate operation. Elias et al ('919) teach that the mixture of silazanes, silica, and triorganosiloxy endblocked polydiorganosiloxane fluid can be mixed to "in situ treat" the silica. The compositions of Elias et al ('919) are those which contain a vinyl containing polysiloxane, the treated silica filler, organohydrogenpolysiloxane crosslinker, and a platinum catalyst. However, Elias et al ('919) do not teach peroxide cured perfluoroalkyl containing polydiorganosiloxanes with improved compression set.
Nitzsche et al in United Kingdom Patent No. 897,305, published May 23, 1962 teach that diorganopolysiloxane elastomers with low compression set can be prepared from diorganopolysiloxane containing a small percentage of vinyl groups (0.05 to 1.0 mol percent), a filler which has been pretreated with an alkenyl silane or a filler which consists of a hydrolyzed and condensed vinyl or allyl trichlorosilane. These elastomers are crosslinked using peroxides or by exposure to high energy radiation.
Maxson in U.S. Pat. No. 4,683,277, issued July 28, 1987, teaches the preparation of liquid diorganovinylsiloxy terminated polydiorganosiloxane of the formula EQU ViR".sub.2 Si(OSiR*R**).sub.P OSiR".sub.2 Vi
in which R* is a monovalent hydrocarbon radical R** is a perfluoroalkylethyl radical, R" is R* or R**, Vi is vinyl and p is 10 to 200. This liquid polymer is prepared by reacting a silanol terminated polydiorganosiloxane liquid with a disilazane to give the terminated liquid polymer.
Shin Etsu Chem Ind KK in Japanese OPI 62/174,260, published July 31, 1987, teaches fluorosilicone rubber which is made from organopolysiloxane which contains vinyl groups on a terminal and reports that the tensile strength and elongation are improved.
Reduction in compression set values of silicone rubber, including fluorosilicone rubber, has been achieved by many routes, such as by the use of metal oxides and carboxylates, and by the introduction of vinyl radical in the polymer chain. Because some of these techniques for compression set reduction are suitable for some types of silicone rubber, they do not provide satisfactory reductions for other types of silicone rubber. While it is known that the compression set values can be reduced by the use of polysiloxanes which contain small amounts of vinyl radical, still further reductions are desirable for fluorosilicone rubber.
Elias et al in U.S. Pat. No. 4,882,368, issued Nov. 21, 1989, teach that the compression set of fluorosilicone rubber can be lowered by endblocking the fluorine-containing polydiorganosiloxane gums used to make the fluorosilicone rubber with alkenyl unsaturation. A method of making the alkenyl endblocked fluorine-containing polydiorganosiloxane gum is to react the hydroxyl endgroups with alkenyl containing silazane compounds. The gum is then mixed with a hydrophobed reinforcing silica filler and an organic peroxide to make a composition which can be cured to a fluorosilicone rubber with a lower compression set than the same composition where the polydiorganosiloxane does not contain the alkenyl endblocking. Elias et al ('368) requires that the polydiorganosiloxanes are first prepared prior to the preparation of the compositions which are cured to make the fluorosilicone rubber. Although this is an acceptable route to low compression set fluorosilicone rubbers, a more flexible method of lowering the compression set is desirable. Lowering the compression set of an otherwise suitable silicone elastomer provides increased flexibility by making a single base applicable for more uses, such as situations where the compression set is not of importance and for applications where the compression set is very important as it is in gasketing.